Power Converter With Capacitive Energy Transfer And Fast Dynamic Response

ABSTRACT

A converter circuit and related technique for providing high power density power conversion includes a reconfigurable switched capacitor transformation stage coupled to a magnetic converter (or regulation) stage. The circuits and techniques achieve high performance over a wide input voltage range or a wide output voltage range. The converter can be used, for example, to power logic devices in portable battery operated devices.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of co-pending U.S. application Ser.No. 13/599,037 filed Aug. 30, 2012, which is a continuation of U.S.application Ser. No. 13/487,781 filed Jun. 4, 2012 now U.S. Pat. No.8,643,347, which is a continuation of application Ser. No. 12/437,599filed May 8, 2009 now U.S. Pat. No. 8,212,541, which claims the benefitof U.S. Provisional Application No. 61/051,476 filed May 8, 2008 under35 U.S.C. §119(e) all of which applications are hereby incorporatedherein by reference in their entireties.

FIELD OF THE INVENTION

The circuits and techniques described herein relate generally to powerconverters and more particularly to power converters having a capacitiveenergy transfer and a fast dynamic response.

BACKGROUND OF THE INVENTION

The advent of portable electronics and low-voltage digital circuitry hasresulted in a need for improved DC-DC converters. DC-DC converters thatcan provide a low voltage output (<2 V) regulated at high bandwidth,while drawing energy from a higher, wide-ranging input voltage (e.g.,typically about a 2:1 range) are particularly useful for supplyingbattery-powered portable electronics. The size, cost, and performanceadvantages of integration make it desirable to integrate as much of theDC-DC converter as possible, including control circuits, power switches,and even passive components. Moreover, it is often desirable, ifpossible, to integrate the power converter or portions thereof with theload electronics.

One common approach is the use of a switched-mode power converter inwhich energy is transferred from the converter input to output with thehelp of intermediate energy storage in the magnetic field of an inductoror transformer. Such magnetics based designs include synchronous buckconverters, interleaved synchronous buck converters, and three-levelbuck converters. Designs of this type can efficiently provide aregulated output from a variable input voltage with high-bandwidthcontrol of the output.

For magnetics-based designs operating at low, narrow-range inputvoltages, it is possible to achieve extremely high switching frequencies(up to hundreds of Megahertz), along with correspondingly high controlbandwidths and small passive components (e.g., inductors andcapacitors). It also becomes possible to integrate portions of theconverter with a microprocessor load in some cases. These opportunitiesarise from the ability to use fast, low-voltage, process-compatibletransistors in the power converter. At higher input voltages and widerinput voltage ranges, much lower switching frequencies (on the order ofa few MHz and below) are the norm, due to the need to use slowextended-voltage transistors (on die) or discrete high-voltagetransistors. This results in much lower control bandwidth, and large,bulky passive components (especially magnetics) which are not suitablefor integration or co-packaging with the devices.

Another conversion approach that has received a lot of attention forlow-voltage electronics is the use of switched-capacitor (SC) basedDC-DC converters. This family of converters is well-suited forintegration and/or co-packaging passive components with semiconductordevices, because they do not require any magnetic devices (inductors ortransformers). An SC circuit includes of a network of switches andcapacitors, where the switches are turned on and off periodically tocycle the network through different topological states. Depending uponthe topology of the network and the number of switches and capacitors,efficient step-up or step-down power conversion can be achieved atdifferent conversion ratios. An example of a step-down SC topology isshown in FIG. 1, which has an ideal conversion ratio M=V_(o)/V_(i)=2.When switches S_(A1) and S_(A2) are closed, the capacitors are chargedin series as illustrated in FIG. 1A, and when switches S_(B1) and S_(B2)are closed, the capacitors are discharged in parallel as illustrated inFIG. 1B.

SC DC-DC converters have been described in prior art literature forvarious conversion ratios and applications, and the technology has beencommercialized. These types of converters have found widespread use inlow-power battery-operated applications, thanks to their small physicalsize and excellent light-load operation.

There are, however, certain limitations of the switched-capacitor DC-DCconverters that have prohibited their widespread use. Chief among theseis the relatively poor output voltage regulation in the presence ofvarying input voltage or load. The efficiency of switched capacitorconverters drops quickly as the conversion ratio moves away from theideal (rational) ratio of a given topology and operating mode. In fact,in many topologies the output voltage can only be regulated for a narrowrange of input voltages while maintaining an acceptable conversion.Another disadvantage of early SC converters is discontinuous inputcurrent which has been addressed in some prior art approaches. These newtechniques, however, still suffer from the same degradation ofefficiency with improved regulation as previous designs.

One means that has been used to partially address the limitations ofswitched-capacitor converters is to cascade a switched capacitorconverter having a fixed step-down ratio with a linear regulator or witha low-frequency switching power converter having a wide input voltagerange to provide efficient regulation of the output. Another approachthat has been employed is to use a switched-capacitor topology that canprovide efficient conversion for multiple specific conversion ratios(under different operating modes) and select the operating mode thatgives the output voltage that is closest to the desired voltage for anygiven input voltage. None of these approaches, however, are entirelysatisfactory in achieving the desired levels of performance andintegration.

A challenge, then, is to achieve the small size and ease of integrationoften associated with SC-based power converters while maintaining thehigh-bandwidth output regulation and high efficiency over a wide inputvoltage range associated with magnetics-based designs.

SUMMARY OF THE INVENTION

A converter circuit and related technique for providing high powerdensity power conversion includes a reconfigurable switched capacitortransformation stage coupled to a magnetic converter (or regulation)stage. One objective of the circuits and techniques described herein isto provide high power density power conversion circuits which convert aninput voltage to an output voltage. In some cases, the circuit convertsinput voltages to output voltages which are lower than the inputvoltages and with a fast transient response. In some cases, the circuitsand techniques can achieve high performance over a relatively wide inputvoltage range. This type of converter can be used to power logic devicesin portable battery operated applications, for example, which oftenexperience wide input voltage ranges. In other cases it may be desirableto operate the power converter circuit so as to provide a relativelywide range of output voltages. This type of converter could be used topower digital circuits with dynamic voltage scaling, or for supplyingpower to polar RF power amplifiers, for example, where wide outputvoltage ranges are commonly required.

Conventional (e.g. magnetic-based) power converters must typicallyemploy semiconductor switches that are rated for voltages similar insize to the input voltage. These relatively high-voltage blockingdevices are inherently slower than lower-voltage devices, and sufferfrom a higher on-state resistance or larger gate capacitance which bothreduce overall efficiency. It would thus be desirable to have aconverter that provides high efficiency and fast regulation of theoutput over a wide input voltage range. Such a converter that combinesthe strengths of the SC techniques (ease of integration, light-loadperformance) with the high efficiency and good regulation ofconventional switched-mode power converters would be a significantimprovement over conventional designs.

In accordance with the circuits and techniques described herein, a powerconverter circuit includes a reconfigurable switched capacitortransformation stage adapted to accept an input voltage at inputterminals thereof and provide power conversion at multiple distinctconversion ratios and provide an intermediate output voltage (or moresimply, an intermediate voltage) at output terminals thereof wherein thetransformation stage is controlled as a function of input voltage suchthat the intermediate voltage is smaller than the input voltage andvaries over a much smaller range (ratio) than the input voltage. Thepower converter circuit further includes a regulation stage coupled toreceive the intermediate output voltage provided by the reconfigurableswitched capacitor transformation stage and to provide an output voltageat a pair of regulation stage output terminals.

With this particular arrangement, a reconfigurable switched capacitorconverter which can provide efficient power conversion at multipledistinct conversion ratios is provided. By providing the transformationstage as a reconfigurable switched capacitor converter with multipletransformation ratios, the intermediate voltage provided to theregulation stage is smaller than the input voltage and varies over amuch smaller range (ratio) than the input voltage. Thus, the regulationstage may operate as a low voltage regulation stage capable ofrelatively high switching frequencies. Thus a converter able to accept arelatively large input voltage range and provide a relatively largeoutput current range is provided. The transformation stage providespower conversion at multiple distinct conversion (or transformation)ratios as a function of input voltage. For example, the conversion ratioof the transformation stage may be selected from among the allowed setof conversion ratios to keep the intermediate voltage as close to adesired reference voltage or as large as possible below some specifiedvoltage. In some cases, the transformation ratio of the transformationstage may also be selected based on a desired or actual system outputvoltage. For example, the reference voltage for the intermediate voltage(at the output of the transformation stage) may be set based on thedesired output system voltage. In these designs, the conversion (i.e.the transformation) ratios are provided as a function of input voltagesuch that an intermediate voltage provided at an output of thetransformation stage varies over a range of voltages which is smaller(ratio) than the range of voltages over which the input voltage varies.

In another aspect of the concepts described herein, by providing thetransformation stage as a reconfigurable switched capacitor converterwith multiple transformation ratios, the converter can better supply arange of output voltages while maintaining high-efficiency operation. Insome cases, the range of output voltages may be narrower than the inputvoltage range while in other cases the range of output voltages may bewider than the input voltage range. The transformation stage providespower conversion at multiple distinct conversion (or transformation)ratios as a function of the desired or actual output voltage. Forexample, the conversion ratio of the transformation stage may beselected from among an allowed or available set of conversion ratios tomaintain an intermediate voltage (e.g. a voltage or range of voltagesprovided at the output of the transformation stage and at the input ofthe regulation stage) as close to a desired factor more than the outputvoltage (e.g., twice the desired output voltage) while not having theintermediate voltage exceed an allowed voltage limit or range on theinput of the regulation stage. In this manner, the regulation stage canoperate over a voltage conversion ratio range that is narrower than therange over which the desired output voltage varies, enabling betterdesign of the regulation stage. In some cases, the transformation ratioof the transformation stage may also be selected based upon the systeminput voltage. In these designs, the conversion (i.e. thetransformation) ratios of the transformation stage are provided as afunction of the output voltage or output voltage reference such that anintermediate voltage provided at an output of the transformation stagevaries over a range of voltages which is smaller (ratio) than the rangeof voltages over which the desired output voltage varies. Thus, whilethe circuit architecture described herein can be used in applicationswhere it is necessary to handle a wide-range-input voltage, the circuitarchitecture described herein is also valuable for handlingwide-range-outputs. Modern digital electronics often operate dynamicallyover a range of output voltages, and thus it may be desirable to providea single converter to handle a range of desired output voltages.

Thus, a controller for providing control signals to switches in one orboth transformation and regulation stages can utilize either or all ofan input voltage, an output voltage and an intermediate voltage todetermine what control signals to provide to switching elements withintransformation or regulation stages and thus control the operation ofthe transformation and regulation stages.

In another aspect, the transformation stage and regulation stage eachinclude two or more switches. The switches in the regulation stage areselected to operate at a switching frequency which is higher than theswitching frequency of the switches in said transformation stage. Thisbenefits the efficiency, power density and control bandwidth of theconverter. For a given switching frequency at small size scales, thepower density of a switched capacitor converter can be much higher thanthat of a magnetic converter. Moreover, a (low-voltage) magneticconverter stage can be efficiently switched at a much higher frequencythan a (high-voltage) switched capacitor converter stage. Thus,operating the SC transformation stage at low frequency and operating themagnetic regulation stage at high frequency provides the bestcombination of system efficiency and power density. Moreover, as theswitching frequency of the regulation stage sets the overall controlbandwidth, increased frequency of the regulation stage as compared tothe transformation stage enables provision of fast transient performancewhile maintaining high efficiency.

In accordance with a further aspect of the circuits and techniquesdescribed herein, a power converter circuit includes a switchedcapacitor circuit coupled to an auxiliary high-frequency converterwherein the high-frequency switching converter switches at a switchingfrequency higher than that of said switched-capacitor circuit andrecovers energy normally dissipated when charging capacitors of saidswitched capacitor circuit.

With this particular arrangement, a switched capacitor converter whichcan provide efficient power conversion at is provided. This switchedcapacitor converter may provide efficient conversion at one conversionratio or in other designs at multiple distinct conversion ratios. Thehigh-frequency switching converter runs at a switching frequency farhigher than that of the switched-capacitor circuit and recovers theenergy that is normally dissipated when charging and/or discharging thecapacitors. The high-frequency switched converter can also providehigh-bandwidth regulation of the converter system output. The circuitcan be configured such that the difference between the input voltage andthe sum of the charging capacitor voltages appears across the inputterminals of the fast switching converter. Because this voltage issubstantially lower than the switched-capacitor circuit input voltage,the auxiliary regulating converter can utilize fast, low-breakdownsemiconductor switches that enable fast operation and regulation. Bymaking the apparent input resistance of the auxiliary converter higherthan the ESR of the switched-capacitor and the SC circuit semiconductorswitch resistances, a majority of the capacitor-charging energy can berecovered. Both of these embodiments allow for a sub 1 V dc output withhigh-bandwidth control of the output.

Thus, described herein are two embodiments, each of which takesadvantage of both switched capacitor and magnetics-based switchingregulator technology. The first embodiment combines a high efficiencyreconfigurable switched capacitor transformation stage with a highfrequency, low-voltage regulation stage. This type of architecture willachieve its highest integration when implemented in a multiple voltagemonolithic process, such as an extended drain process. The devices inthe switched capacitor network need not block the full supply voltage,and depending upon the configuration, they need to block differentfractions of the supply voltage. Similarly, high speed low voltagedevices can be used in the switching regulator since it only sees avoltage slightly above the output voltage. Thus, the efficiency of thefull converter can be optimized by using different voltage devices.

The second embodiment tightly couples a switched capacitor circuit withan auxiliary high-frequency converter (e.g. a low-voltage magneticsbased converter). The high-frequency switching converter runs at aswitching frequency far higher than that of the switched-capacitorcircuit and recovers the energy that is normally dissipated in parasiticresistances when charging and/or discharging the capacitors. Thehigh-frequency switched converter can also provide high-bandwidthregulation of the converter system output. The circuit can be configuredsuch that the difference between the input voltage and the sum of thecharging capacitor voltages appears across the input terminals of thefast switching converter. Because this voltage is substantially lowerthan the switched-capacitor circuit input voltage, the auxiliaryregulating converter can utilize fast, low-breakdown semiconductorswitches that enable fast operation and regulation. By making theapparent input resistance of the auxiliary converter higher than theequivalent series resistance (ESR) of the switched-capacitor and the SCcircuit semiconductor switch resistances, a majority of the energyusually lost in capacitor-charging can be recovered. Both of theseembodiments allow for a sub 1 V dc output with high-bandwidth control ofthe output.

In accordance with a still further aspect of the concepts describedherein, an integrated CMOS circuit includes a reconfigurable switchedcapacitor transformation stage having a transformation stage input portand a transformation stage output port and comprising one or more CMOSswitches and one or more discrete or integrated storage elements, saidtransformation stage configured to accept an input voltage at thetransformation stage input port and provide an intermediate voltage atthe transformation stage output port; and a regulation stage having aregulation stage input port and a regulation stage output port andcomprising one or more CMOS switches implemented as base transistors ofa CMOS process and one or more discrete or integrated storage elements,said regulation stage configured to accept the intermediate voltageprovided by said reconfigurable switched capacitor transformation stageand configured to provide an output voltage at the regulation stageoutput port. In a preferred embodiment, the switches in said CMOSreconfigurable switched capacitor transformation stage and said CMOSregulation stage are provided in a single CMOS process. It should beappreciated that in some embodiments, storage elements (e.g. capacitors)in one or both of the reconfigurable switched capacitor transformationstage and/or the regulation stage may not be integrated onto theintegrated circuit while in other embodiments, the storage elements inone or both of the reconfigurable switched capacitor transformationstage and the regulation stage may be integrated onto the integratedcircuit along with the switches.

In accordance with a still further aspect of the concepts describedherein a power converter circuit includes a switched capacitor circuithaving a switched-capacitor input port and a switched capacitor outputport, said switched-capacitor circuit comprising a plurality of switchesand one or more capacitors, said switched-capacitor circuit switchingthe capacitors between at least two states to transfer energy from theswitched-capacitor input port to the switched-capacitor output port, andan auxiliary converter stage coupled to said switched capacitor circuitwherein said auxiliary converter stage switches at a switching frequencyhigher than that of said switched-capacitor circuit such that saidauxiliary converter recovers energy normally dissipated when charging ordischarging capacitors of said switched capacitor circuit.

In one embodiment, the auxiliary converter recovers energy normallydissipated when charging capacitors in said switched capacitor circuitby absorbing an instantaneous difference between stacked capacitorvoltages in said switched capacitor circuit and an input voltage or anoutput voltage of the power converter circuit.

In one embodiment, the auxiliary converter stage comprises a pluralityof switches, and one or more magnetic energy storage components and theregulation stage switches at a frequency which is at least five timesthat of the switching frequency of the switched capacitor circuit.

In one embodiment, the switched capacitor circuit and said auxiliarycircuits are provided as CMOS circuits. In a preferred embodiment, theswitches in the CMOS switched capacitor circuit and CMOS auxiliarycircuits are provided in a single integrated process. In a preferredembodiment, the switches in the CMOS auxiliary circuits are implementedwith base transistors of a CMOS process.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing features of the circuits and techniques described herein,may be more fully understood from the following description of thedrawings in which:

FIG. 1 is a schematic diagram of a prior art switched-capacitor circuit;

FIG. 1A is a schematic diagram of a prior art switched-capacitor circuitin a first state with capacitors charging in series;

FIG. 1B is a schematic diagram of a prior art switched-capacitor circuitin a second state with capacitors discharging in parallel;

FIG. 2 is a block diagram of a converter having a transformation stageand a regulation stage;

FIG. 3 is a schematic diagram of a reconfigurable switched-capacitorconverter having a plurality of capacitors;

FIG. 3A is a schematic diagram of a switched-capacitor circuit operatingat a conversion ratio of one-third (⅓);

FIG. 3B is a schematic diagram of a switched-capacitor circuit operatingat a conversion ratio of one-half (½);

FIG. 3C is a schematic diagram of a switched-capacitor circuit operatingat a conversion ratio of two-thirds (⅔);

FIG. 4 is a plot of output voltage vs. input voltage of an SC converterhaving three distinct step-down ratios (⅓, ½, ⅔) and a predeterminedmaximum output voltage;

FIG. 5 is a schematic diagram illustrating charging of a capacitor withresistive power loss;

FIG. 5A is a schematic diagram illustrating charging of a capacitor withenergy recovery by auxiliary converter;

FIG. 6 is a schematic diagram of a power converter circuit whichincludes a switched-capacitor converter stage coupled to a regulatingconverter stage;

FIG. 7 is a plot of switch configuration vs. voltage; and

FIG. 8 is a schematic diagram of a power converter circuit whichincludes a switched-capacitor converter stage coupled to a regulatingauxiliary converter stage which provides a separate output voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before describing several exemplary embodiments of power convertercircuits and processing performed by and on such power convertercircuits, it should be appreciated that, in an effort to promote clarityin explaining the concepts, reference is sometimes made herein tospecific switched capacitor circuits or specific switched capacitorcircuit topologies. It should be understood that such references aremerely exemplary and should not be construed as limiting. After readingthe description provided herein, one of ordinary skill in the art willunderstand how to apply the concepts described herein to providespecific switched capacitor (SC) circuits or specific switched capacitorcircuit topologies. For example, while series-parallel SC topologies maybe disclosed herein, such disclosure is provided to promote clarity inthe description of the general concepts described herein. After readingthe disclosure provided herein those of ordinary skill in the art willappreciate that a series-parallel SC topology is only one of manypossible topologies. It should thus be understood that although specificswitched capacitor circuits or specific switched capacitor circuittopologies are not specifically disclosed herein, such circuits stillfall within the scope of the concepts claimed herein.

It should be appreciated that reference is also sometimes made herein toparticular input, output and/or intermediate voltages and/or voltageranges as well as to particular transformation values and or ranges oftransformation values. It should be understood that such references aremerely exemplary and should not be construed as limiting.

Reference is also sometimes made herein to particular applications. Suchreferences are intended merely as exemplary should not be taken aslimiting the concepts described herein to that particular application.

Reference is also sometimes made herein to circuits having switches orcapacitors. Its should be appreciated that any switching elements orstorage elements having appropriate electrical characteristics (e.g.appropriate switching or storage characteristics) may, of course, alsobe used.

Thus, although the description provided herein below explains theinventive concepts in the context of a particular circuit or aparticular application or a particular voltage or voltage range, thoseof ordinary skill in the art will appreciate that the concepts equallyapply to other circuits or applications or voltages or voltage ranges.

Referring now to FIG. 2, a power converter circuit 10 includes a firststage 12 (also referred to as a “reconfigurable switched capacitortransformation stage” or a “switched capacitor stage” or more simply a“transformation stage”) and a second stage 14 (also referred to asswitching converter regulation stage or more simply a “regulationstage”). A voltage source 15 (here shown in phantom since it is notproperly a part of the power converter circuit 10) is coupled between apair of input terminals 12 a, 12 b of the transformation stage of powerconverter circuit 10 and a load R_(L) (also shown in phantom since it isnot properly a part of the power converter circuit 10) coupled to outputterminals 14 a, 14 b of regulation stage 14 and across which isgenerated an output voltage V_(O). A controller circuit 16 is coupled toreceive a reference voltage V_(REF), as well as some or all of the inputand output voltages V_(IN) and/or V_(OUT) and an intermediate outputvoltage V_(x) (or more simply, intermediate voltage V_(x)). Controller16 receives signals provided thereto (e.g. any or all of V_(IN), V_(OUT)and/or intermediate voltage V_(x)) and in response thereto (and inaccordance with a desired operating mode) provides control signals onpaths 17 a, 17 b to either or both of the transformation and regulationstages 12, 14, respectively.

Transformation stage 12 receives the input voltage (e.g. V_(IN).) andoperates to provide a transformed or intermediate voltage V_(X) atterminals 12C-12D. Thus transformed voltage V_(X) is provided to inputterminals of regulation stage 14.

It should be appreciated that the input voltage V_(IN) may vary over arelatively wide voltage range. The particular voltage range over whichthe input voltage may vary depends upon the particular application. Forexample, in some applications the range of input voltages may be fromabout 1.5 volts (V) to about 5.0V. In other applications the range ofinput voltages may be from about 6V to about 12V. In still otherapplications the input voltage range may be from about 10V to about 14V.For example, in a converter circuit for battery-powered portableelectronics applications, operation may be typically be required acrossan input voltage range from 2.4 V to 5.5 V.

Regardless of the input voltage, however, the transformation stage 12,maintains transformed voltage V_(X) over a voltage range which isrelatively narrow compared with the input voltage range. For example, inthe case where the input voltage range is from about 1.5 volts (V) toabout 5.0V, the output voltage of the transformation stage 12 may rangefrom about 1.0V to about 1.66V. Furthermore, the transformation ratiosutilized by the transformation stage 12 are selected as a function ofthe input voltage V_(IN). For example, the conversion ratio of thetransformation stage may be dynamically selected from among the allowedset of conversion ratios such that the intermediate voltage V_(x) willbe as large as possible while remaining below a specified maximumvoltage. Thus, by adjusting a transformation ratio, transformation stage12 can accept a wide range of input voltages while maintaining thetransformed voltage over a voltage range which is relatively narrowcompared with the input voltage range. For example, consider atransformation stage an input voltage range of 1.5 to 5.0 V and havingallowed conversion ratios of ⅓, ½, and ⅔. It is possible to meet a goalof maximizing an intermediate voltage while at the same time keeping itbelow a specified maximum of approximately 1.66 V by operating at aconversion ratio of ⅔ for input voltages from 1.5 V to 2.5 V, operatingat a conversion ratio of one half for input voltages from 2.5 V to 3.33V, and operating at a conversion ratio of ⅓ for input voltages from 3.33V to 5 V.

The transformation stage 12 and regulation stage 14 each include one ormore switch components and one or more energy storage components. Thecomponents which provide the transformation stage 12 are selected suchthat the transformation stage has a switching frequency which isrelatively low compared with the switching frequency of the regulationstage. Thus, the transformation stage may be referred to a low frequencystage while the regulation stage may be referred to as a high frequency,low voltage magnetic stage. The difference in switching speeds of thetransformation stage and regulation stage switches (i.e. the frequencyseparation between the switching frequencies of the switches) isselected based upon a variety of factors including but not limited tothe gating and switching loss characteristics of the switches It should,of course, be appreciated that a tradeoff must be made between switchingfrequency and the voltage levels (and/or range of voltages) which mustbe accepted by and provided by the transformation and regulation stages.

Transformation stage 12 includes a first plurality of coupled switches18, 20, 22 coupled between terminal 12 a and a terminal 12 c.

A first capacitor 24 has a first terminal coupled to a first terminal ofswitch 18 and a second terminal coupled to a first terminal of a switch26. A second terminal of switch 26 is coupled to an interconnect path 27coupled between terminal 12 b and a terminal 12 d. In this particularembodiment, interconnect path 27 is coupled to a negative terminal ofthe voltage source 16.

A first terminal of a switch 28 is coupled to the second terminal ofcapacitor 24 and a second terminal of switch 28 is coupled to a nodebetween switches 20 and 22.

A second capacitor 30 has a first terminal coupled to a first terminalof switch 22 and a second terminal coupled to a first terminal of aswitch 32. A second terminal of switch 32 is coupled to the interconnectpath coupled between terminal 12 b and terminal 12 d.

A first terminal of a switch 34 is coupled to the second terminal ofcapacitor 30 and a second terminal of switch 34 is coupled to a secondterminal of switch 22. Thus, by proper operation of switches 18, 20, 22,26, 28, 32 and 34, capacitors 24 and 30 may be selectively coupled inparallel between terminals 12 c and 12 d. Alternatively, by properoperation of switches 18, 20, 22, 26, 28, 32 and 34, capacitors 24 and30 may be selectively coupled in parallel with switches 20, 22,respectively, creating a series stack between terminal 12 a and terminal12 c.

It should, of course, be appreciated that the above-described operatingmode is merely one of a plurality of different possible operating modesin a reconfigurable circuit. Another exemplary operating mode (orcircuit configuration) is illustrated in FIG. 3B. The system dynamicallyselects between a plurality of possible patterns and thus is said to bedynamically reconfigurable.

The transformation stage 12 corresponds to a reconfigurable switchedcapacitor converter. By appropriately selecting the switching patternsof the switches from among the possible patterns, the switched capacitorconverter is reconfigurable and thus is able to provide efficient powerconversion at multiple distinct conversion ratios. The operating mode ofthe transformation stage 12 is controlled as a function of inputvoltage. For example, for large input voltages one can operate thecircuit to follow the switching patterns in FIG. 3A, providing 3:1conversion, while for lower input voltages one can operate the circuitto follow the switching patterns of FIG. 3 b, thus providing 2:1conversion. In this way, transformation stage 12 can efficiently providean intermediate voltage V_(X) between terminals 12 c, 12 d that issmaller than the input voltage V_(IN) and which varies over a muchsmaller range (ratio) than the input voltage V_(IN).

The second, or regulating, stage 14 corresponds to a magnetic-basedswitching power converter which operates from the low, narrow-range(i.e. any range less than the input voltage range; for example, if theratio of V_(IN)/V_(O) equals 2:1 then anything less than that would beconsidered narrow-range) intermediate voltage to regulate the outputvoltage V_(o). As this stage operates from a relatively low, narrowinput voltage range, it can be designed to operate at relatively highfrequencies. Since component size is related to switching frequency(e.g. the higher the switching frequency, the smaller the component),the circuit may be implemented using passive components which arerelatively small in size and which provide high-bandwidth regulation ofthe output. Thus, power converter 10 converts power in two stages (i.e.the transformation stage 12 and the regulating stage 14) and together,the two stages can provide very small size, high efficiency, and highcontrol bandwidth characteristics.

The transformation or switched-capacitor stage 12 is designed (e.g. byinclusion of multiple switched capacitor building blocks) to efficientlyconvert power at multiple distinct voltage conversion ratios. The numberof capacitors in the converter stage determines both the maximumconversion ratio and the total possible number of distinct conversionratios. It should thus be appreciated that the particular number ofswitches and capacitors included in the transformation stage dependsupon a variety of factors including but not limited to the input voltagerange for a particular application and the output voltage required for aparticular application, and how many different transformation ratios aredesired to reduce the intermediate voltage range.

The power converter circuit described herein may be fabricated as anintegrated circuit using a CMOS process. In this case, an integratedCMOS circuit includes a reconfigurable switched capacitor transformationstage provided from one or more CMOS switches and one or more storageelements, in which the storage elements may be realized as eitherintegrated capacitors or external devices. The regulation stage wouldalso be provided from one or more CMOS switches implemented and one ormore storage elements. Again, the storage elements may be integrated,discrete, or provided as bond wires. In a preferred embodiment, theswitches in said CMOS reconfigurable switched capacitor transformationstage and said CMOS regulation stage are fabricated in a single CMOSprocess. In the case where it is desirable for the switches of theregulation stage to switch at a higher frequency (in some cases asignificantly higher frequency) than the switches in the transformationstage, the switches of the regulation stage may be fabricated as basetransistors in the CMOS process.

Referring now to FIGS. 3-3C in which like elements are provided havinglike reference designations, a transformation stage 30 (also referred toas a switched capacitor stage) includes eight switches M1-M8 and threecapacitors C1-C3. The switches may be selectively opened and closed asshown in Table 1 below to provide three distinct conversion ratios (inthis case, step-down ratios) of: ⅓; ½; and ⅔.

TABLE 1 V₂/V₁ M₁ M₂ M₃ M₄ M₅ M₆ M₇ M₈ M₉ 1/3 clk clk clk clk clk clk clkoff off 1/2 clk clk clk clk on on off off off 2/3 clk clk clk off offclk clk clk clk V_(max) $\frac{2}{3}V_{1}$ $\frac{2}{3}V_{1}$$\frac{1}{2}V_{1}$ $\frac{1}{2}V_{1}$ $\frac{2}{3}V_{1}$$\frac{1}{3}V_{1}$ $\frac{1}{2}V_{1}$ $\frac{1}{2}V_{1}$$\frac{1}{2}V_{1}$The SC transformation stage is provided with a digital clock signal clk.A second signal /clk is also generated, which may simply be thecomplement of clk (i.e., is high when clk is low and low when clk ishigh), or which may be generated as a non-overlapping complement as iswell known in the art. The elements of the first three rows of table 1indicate the switching states of the individual switches as the circuitis clocked. Each row shows operation for a different conversion ratio(i.e., operating configuration). An entry clk indicates that the switchis on (closed) when clk is asserted and off (open) otherwise, an entry/clk indicates that the switch is on when the complementary signal /clkis asserted and off otherwise, an entry off indicates that the switch isalways off for that conversion ratio, and an entry on indicates that aswitch is always on for that conversion ratio.

Referring now to FIG. 3A, with a switching pattern set in accordancewith the second row of Table 1, the switched-capacitor circuit 30provides a step down ratio of one-third (⅓).

Referring now to FIG. 3B, with a switching pattern set in accordancewith the third row of Table 1, the switched-capacitor circuit 30provides a step down ratio of one-half (½).

Referring now to FIG. 3C, with a switching pattern set in accordancewith the third row of Table 1, the switched-capacitor circuit 30provides a step down ratio of two-thirds (⅔).

It should be appreciated that the maximum voltage to which any of thedevices in transformation stage 30 will be exposed is ⅔ of the inputvoltage (V₁) and some of the devices may see less (½V₁ and ⅓V₁)depending upon a selected operating mode. It should be appreciated thatall of the devices in transformation stage 30 must be able to block therequired voltage to realize all three step-down ratios. For example, M₁must be rated to block ⅔V₁, however it only sees ½V₁ for a step-downratio of ½ and ⅔V₁ for step-down ratios of ⅓ and ⅔. If more complicatednetworks are used (i.e. more switches), it may be possible to realizethe same functionality at higher efficiency, but at the cost of area.

The switched capacitor stage 30 is thus controlled to maintain theintermediate voltage V_(x) within a specified (narrow) range (or window)of voltages as the input voltage varies across a wide range of voltages.The size, maximum voltage and minimum voltage of the output window ofthe SC transformation stage can be tailored to fit the regulation stage.

Referring now to FIG. 4, one possible windowing scheme fortransformation stage 30 (a/k/a switched capacitor stage 30) of FIG. 3 isshown. Transformation stage 30 can accept an input voltage in the rangeof about 1.5V to about 5.0V (3.33:1) and convert it into window of about0.66 V ranging from about 1V to about 1.66V. Various control schemes canbe used to accomplish this task. That is, the particular manner in whichswitches in transformation stage 30 (FIG. 3) are selectively opened andclosed to provided a desired transformation ratio may be selected basedupon a variety of factors including but not limited to how charge flowsin the circuit and how much loss is generated by a particular pattern.

As is understood in the art, the circuit topology and switching patternsare selected such that charge balance is maintained on the chargestorage elements (e.g. capacitors), which imposes a rational conversionratio between the input and the output charge (and current) for a givenswitching pattern. This conversion ratio is the inverse of the idealvoltage conversion ratio of the circuit for a given switching pattern.In region 32, a transformation ratio of ⅔ is used while in region 34 atransformation ratio of ½ is used while in region 36 a transformationration of ⅓ is used. It should be appreciated that the waveform of FIG.4 is selected strictly on V_(IN), but it should be appreciated that theselection could be made based upon both V_(IN) and V_(OUT). For example,one might select the switching pattern transitions and hence theintermediate voltage window so that the regulation stage can operatenear a desired conversion ratio, maximizing efficiency.

The regulating stage can be implemented with numerous topologies forvery low output voltages (e.g. sub 1 V); good options include asynchronous buck converter, cascode-switch synchronous buck converter,interleaved synchronous buck converter, three-level synchronous buckconverter, and four-switch “buck-boost” converter. Very fasttransistors, such as base CMOS transistors in an integrated process, canbe used in the regulation stage since the input voltage is quite low.This allows the regulation stage to operate at a very high switchingfrequency, which in turn reduces the size of its passive components.

One option to achieve a very high degree of integration is to fabricatethe converter in a multiple-voltage monolithic process (e.g., a processproviding for extended drain transistors). The switched capacitor stagecan be implemented with higher voltage devices and operated at arelatively low frequency (e.g., 1 MHz) commensurate with thehigh-voltage devices. The regulating stage can be implemented withlow-voltage devices, and thus can be operated at considerably higherfrequencies (e.g., 100 MHz), providing small volume for the passivecomponents and fast regulation. Alternatively, multi-chip fabricationcan be used in which the two stages are implemented in differentprocesses, each optimized for their respective functions.

A converter with a large input voltage range and output current rangecan thus be realized if a reconfigurable switched capacitor converterwith multiple transformation ratios is used as a transformation stagealong with a high frequency, low voltage regulation stage. The convertercan also take advantage of state-of-the-art CMOS processes that haveadditional high voltage devices.

The energy loss E_(L) associated with charging a capacitor C from zeroto a voltage V with a series connection from a dc voltage source ofvalue V is ½ CV², and is independent of the parasitic series resistance(R). Furthermore, for a conventional SC circuit, a fixed amount ofcharge-up energy loss equal to ½ CΔV² will result at each switchinterval, where ΔV corresponds to the difference between the initial andfinal value of the capacitor voltage. It is important to note that thisfixed charge-up loss cannot be reduced by employing switches with loweron-state resistance. It is for this reason that conventional SCconverters aim to minimize the variations of the voltage on thecapacitors during the charging phase and only operate efficiently atcertain conversion ratios. Consequently, conventional SC convertersrequire relatively large capacitors to achieve high efficiency and powerlevels and do not provide efficient regulation from variable inputvoltages. As is shown below, a second embodiment of a power convertercircuit permits more efficient use of the capacitors, enabling reductionin the required capacitor size and/or improvement in system efficiency.Furthermore, the second embodiment does not require a reconfigurableswitched-capacitor network although it may use one.

To understand the approach used in the second embodiment, consider thecircuit of FIG. 5 which is a simple example, which illustrates theloss-mechanism for charging of the capacitors in the switched capacitorstage. FIG. 5 is used to explain an example of the charging process of acapacitor C, where a resistor R represents the combined equivalentseries resistance (ESR) of the capacitor and switch on-state resistance.The capacitor has an initial charge of V_(i), and the switch is closedat t−0⁺ After t=0⁺, the difference between voltage V_(in) and thecapacitor at each instance in time appears across the parasitic resistorR resulting in dissipation during charging. If charging is allowed tocontinue for a sufficient period of time, the voltage across thecapacitor will charge up to V_(in), and the voltage across the resistorwill become 0 V. The voltage across the resistor and the current throughit, results in a power loss during the charging phase of the capacitor,which depends on the capacitance and the net charge in the capacitor. Itis this loss which limits the efficiency of the switched capacitorstage.

FIG. 5A illustrates a technique to improve the charge-up efficiency ofthe switched capacitor circuit. In this embodiment, an auxiliaryconverter 40 operating at a much faster switching frequency than theswitched capacitor stage is used to reduce the energy loss of theswitched capacitor circuit. The switching frequency of the auxiliaryconverter should be sufficiently higher than the switched capacitorstage such that the capacitor charging takes place over many switchingcycles of the auxiliary converter. A factor of five in switchingfrequency may be sufficient for this purpose, and factors of ten or moreare typical.

The auxiliary converter 40 may be the regulating converter used tosupply the output, or it may be a separate converter. The system isdesigned such that the majority of the difference between the inputvoltage V_(in) and the capacitor stack voltage V_(C) appears across theinput of the auxiliary converter when the capacitor is charging. Insteadof being dissipated as heat in the resistor, the energy associated withcharging the capacitor stack is delivered to the output of the auxiliaryconverter.

FIG. 6 illustrates a possible implementation of the general embodimentdiscussed above in conjunction with FIG. 5.

Referring now to FIG. 6, a power converter 50 includes a transformationstage 52 and an auxiliary converter and the regulating converter stage54. Transformation stage 52 includes a plurality of capacitors C1, C2and a first group of switches (each labeled S1) and a second group ofswitches (each labeled S2). Auxiliary converter and the regulatingconverter stage 54 includes a plurality of capacitors C_(small),C_(buck), a plurality of switches S3, S4 and an inductive elementL_(buck).

In the exemplary embodiment shown in FIG. 6, a “fast regulatingconverter” (in this case a synchronous buck converter) serves as boththe auxiliary converter and the regulating converter stage 54 for thesystem 50. Auxiliary converter and the regulating converter stage 54operates at a switching frequency much higher than that of the switchedcapacitor stage 52. As the capacitor C_(small) serves only as a filterand bypass for the fast regulating converter, its numerical value can bemuch smaller than the capacitors C₁ and C₂ of the switched capacitortransformation stage. When the switched-capacitor stage is configuredfor charging of C₁ and C₂ (switches S1 closed), the difference betweenV_(in) and the sum of the voltages across capacitors C₁ and C₂ appearsacross the input terminal of the fast regulating converter. C₁ and C₂thus charge with low loss, and at a rate determined by the power drawnfrom the regulating converter to control the system output. Likewise,when the switched-capacitor stage is configured for discharging C₁ andC₂ in parallel (switches S2 closed), the discharge is at a rate based onthe power needed to regulate the output.

In operating the system, the switched capacitor stage 52 can be switched(switch sets S1 and S2 on alternately) such that the voltage V_(x) atthe input of the fast regulating converter stays within a specifiedwindow or below a specified voltage. For example, the switches could becontrolled such that the capacitor C₁ and C₂ voltages remain within aspecified hysteresis band about V_(in)/3, such that the regulatingconverter sees a maximum input voltage near V_(in)/3. Alternatively, theswitched capacitor stage can be controlled to provide a specifiedmaximum voltage V_(x) at the input of the auxiliary converter.

Referring now to FIG. 7, a plot of switch configuration vs. voltageillustrates a control strategy utilizing the above-described technique,where two separate reference voltages are used to ensure that the inputvoltage of the auxiliary converter does not exceed V_(X,max). In thisexample, the switches designated by reference number 1 in FIG. 6 are onor closed (series charging of the capacitors) until V_(X) falls belowV_(ref1). At this time, switches 1 turn off or open, and the switchesdesignated with reference numeral 2 turn on (parallel discharging ofcapacitors), until V_(X) falls below V_(ref2), at which time the cyclerepeats. The reference voltages are set by the maximum auxiliary inputvoltage V_(X,mas), and are given by:

$V_{{ref}\; 1} = {V_{IN} = {{2\; V_{X,{mas}}\mspace{31mu} V_{{ref}\; 2}} = \frac{V_{in} - V_{X,\max}}{2}}}$

It should be understood that the topology of the fast regulatingconverter (a buck converter in the example of FIG. 6) can be any type ofpower converter that is able to provide fast switching and efficientregulation of the output voltage for various input voltages (synchronousbuck, three-level synchronous buck, SEPIC, soft switched or resonantconverter, etc.). Similarly, the switched-capacitor circuit can berealized with a variety of topologies, depending on desired voltagetransformation and permitted switch voltages.

Referring now to FIG. 8, yet another embodiment of a power convertercircuit is shown. The circuit topology shown in FIG. 8 illustrates howthe auxiliary converter can be used as a separate energy-recoveringdevice, with an independent output voltage. In this embodiment, twoparallel switched-capacitor circuits are employed, operating inanti-phase. When the capacitors of step-down cell 1 are charging inseries (switches A closed, switches B open), the capacitors of cell 2are discharging in parallel. The difference between the input voltageand the sum of the charging capacitors appears across the input terminalof the auxiliary converter. The charging energy can again be recoveredby this auxiliary converter, providing a means for improving overallefficiency or increasing power density. It is important to note that theoutput voltage of the switched-capacitor stage, V_(out1), can beregulated using the auxiliary converter. The auxiliary converterperforms the regulating functions in a manner similar to some (lossy)current-controlled strategies. The auxiliary converter can again be anytype of fast regulating converter, just as in the example of FIG. 6. Theregulated output voltage V_(out2) can be higher or lower than V_(out1),depending on the choice of auxiliary converter. This voltage can be usedfor a variety of purposes. Examples include, but are not limited to:powering the SC transistors, fed back to the input, delivered to theoutput, or providing a separate output voltage for applications wherethat is desirable. R_(load) in this circuit can represent the actualload, or may represent the input impedance of another converter or setof converters for regulating system outputs.

Having described one or more preferred embodiments of the circuits,techniques and concepts described herein, it will now become apparent tothose of ordinary skill in the art that other embodiments incorporatingthese circuits, techniques and concepts may be used. Accordingly, it issubmitted that that the scope of the patent should not be limited to thedescribed embodiments, but rather, should be limited only by the spiritand scope of the appended claims.

What is claimed is:
 1. A converter circuit comprising: a switchedcapacitor transformation stage having a transformation stage input portand a transformation stage output port, said switched capacitortransformation stage comprising one or more switches and one or morecapacitors and said switched capacitor transformation stage configuredto accept an input voltage at the transformation stage input port andprovide an intermediate output voltage at the transformation stageoutput port; an auxiliary converter stage having an input coupled tosaid switched capacitor transformation stage, said auxiliary converterstage comprising one or more switches and one or more magnetic energystorage components configured to recover energy normally dissipated incapacitors of said switched capacitor transformation stage via said oneor more magnetic energy storage components.
 2. The converter circuit ofclaim 1 wherein said auxiliary converter stage operates to recoverconverter stage energy normally dissipated when charging capacitors ofsaid switched capacitor transformation stage.
 3. The converter circuitof claim 1 wherein the input of said auxiliary converter stage isprovided having an apparent input resistance which is higher than anequivalent series resistance of said switched-capacitor transformationstage.
 4. The converter circuit of claim 1 wherein said switchedcapacitor transformation stage operates such that a portion of adifference between the input voltage V_(IN) and a capacitor stackvoltage V_(C) appears across the input of said auxiliary converterstage.
 5. The converter circuit of claim 4 wherein said switchedcapacitor transformation stage operates such that the portion of thedifference between the input voltage V_(IN) and the capacitor stackvoltage V_(C) which appears across the input of the auxiliary converterstage corresponds to a majority of the difference between the inputvoltage V_(IN) and the capacitor stack voltage V_(C) which appearsacross the input of the auxiliary converter stage.
 6. The convertercircuit of claim 1 wherein in response to the one or more switches ofsaid switched capacitor transformation stage being configured so that acapacitor in said switched capacitor transformation stage is charging ordischarging, said switched capacitor transformation stage operates suchthat a portion of the difference between the input voltage V_(IN) and acapacitor stack voltage V_(C) appears across the input of said auxiliaryconverter stage and said auxiliary converter stage recovers energynormally dissipated when charging or discharging capacitors of saidswitched capacitor transformation stage.
 7. The converter circuit ofclaim 13 wherein the portion of the difference between the input voltageV_(IN) and the capacitor stack voltage V_(C) corresponds to a majorityof the difference between the input voltage V_(IN) and the capacitorstack voltage V_(C).
 8. The converter circuit of claim 1 wherein saidone or more switches in said auxiliary converter stage operate at aswitching frequency which is higher than the switching frequency of theone or more switches in said switched capacitor transformation stage. 9.A power converter circuit comprising: a reconfigurable switchedcapacitor transformation stage having a transformation stage input portand a transformation stage output port, said transformation stageconfigured to accept an input voltage at the transformation stage inputport and provide an intermediate output voltage at the transformationstage output port and wherein said reconfigurable switched capacitortransformation stage provides power conversion at multiple distinctconversion ratios as a function of at least one of: input voltage, areference voltage, the intermediate voltage and an output voltage ofsaid power converter circuit; an auxiliary converter stage coupled tosaid reconfigurable switched capacitor transformation stage, saidauxiliary converter stage configured to operate such that said auxiliaryconverter stage recovers energy normally dissipated when charging ordischarging capacitors of said switched capacitor circuit.
 10. The powerconverter circuit of claim 9 wherein: said reconfigurable switchedcapacitor transformation stage comprises one or more switches and one ormore capacitors; and said auxiliary converter stage comprises one ormore switches and one or more magnetic energy storage components. 11.The power converter circuit of claim 9, wherein said reconfigurableswitched capacitor transformation stage is dynamically reconfigurable toprovide energy transfer at multiple, different, conversion ratios. 12.The power converter circuit of claim 9, wherein said auxiliary converterstage recovers energy normally dissipated when charging capacitors insaid switched capacitor circuit by absorbing a difference betweenstacked capacitor voltages in said switched capacitor circuit and afunction of one or more of: an input voltage and an output voltage ofthe power converter circuit.
 13. The power converter circuit of claim 12wherein: said auxiliary converter transfers energy from saidswitched-capacitor circuit to generate a regulated output voltage. 14.The power converter circuit of claim 10 wherein said one or moreswitches in said reconfigurable switched capacitor transformation stagesare disposed to couple said at least one capacitor in series or inparallel with respect to the output port of said reconfigurable switchedcapacitor transformation stage.
 15. A power converter circuitcomprising: a switched capacitor circuit having a switched-capacitorinput port and a switched capacitor output port, said switched-capacitorcircuit comprising one or more switches and one or more capacitors, saidswitched-capacitor circuit switching the one or more capacitors betweenat least two states to transfer energy from the switched-capacitor inputport to the switched-capacitor output port, and an auxiliary converterstage coupled to said switched capacitor circuit wherein said auxiliaryconverter stage switches at a frequency such that said auxiliaryconverter stage recovers energy normally dissipated when charging ordischarging capacitors of said switched capacitor circuit.
 16. The powerconverter circuit of claim 15, wherein said switched capacitor circuitprovides power conversion at multiple distinct conversion ratios afunction of at least one of: an input voltage, a reference voltage andan intermediate voltage.
 17. The power converter circuit of claim 16,wherein said switched capacitor circuit is dynamically reconfigurable toprovide energy transfer at the multiple conversion ratios.
 18. The powerconverter circuit of claim 16, wherein said auxiliary converter stagerecovers energy normally dissipated when charging capacitors in saidswitched capacitor circuit by absorbing an difference between stackedcapacitor voltages in said switched capacitor circuit and a function ofone or more of: an input voltage and an output voltage of the powerconverter circuit.
 19. The power converter circuit of claim 16 wherein:said auxiliary converter stage comprises one or more switches, and oneor more magnetic energy storage components, said auxiliary converterstage switching at a frequency higher than that of said switchedcapacitor circuit.
 20. The power converter circuit of claim 19 wherein:said auxiliary converter stage transfers energy from saidswitched-capacitor circuit to generate a regulated output voltage.
 21. Apower converter circuit having an input port and an output port andcomprising: a reconfigurable switched capacitor transformation stagehaving a transformation stage input port and a transformation stageoutput port, said reconfigurable switched capacitor transformation stagecomprising one or more switches and one or more capacitors configured toaccept a first voltage at the input port of said reconfigurable switchedcapacitor transformation stage and to provide a second voltage at theoutput port of said reconfigurable switched capacitor transformationstage and wherein said reconfigurable switched capacitor transformationstage provides power conversion at multiple distinct conversion ratiosas a function of at least one of: the first voltage, the second voltage,and a reference voltage; and an auxiliary converter stage having anauxiliary converter stage input port and an auxiliary converter stageoutput port, said auxiliary converter stage connected in series withsaid reconfigurable switched capacitor transformation stage between theinput port and the output port of the power converter circuit.
 22. Theconverter circuit of claim 1 wherein said switched capacitortransformation stage provides power conversion at multiple distinctconversion ratios.